Radiation detectors, e.g., detectors capable of detecting X-rays and/or gamma rays, have been developed over the years for a variety of applications, e.g., medical imaging and detection, non-destructive testing and security inspection. Some early detectors included a collimator, a scintillation crystal and a plurality of photomultiplier tubes (PMTs). To overcome some shortcomings associated with PMT detectors, direct conversion detectors have been developed. Direct conversion detectors are capable of operating in photon counting mode or current mode.
Direct conversion radiation detectors, e.g., radiation detectors using Cadmium Zinc Telluride (CZT) or some other direct conversion material, have been developed over the years for a variety of applications. Nearly all of the research on CZT has been devoted to photon counting applications. Typically, these photon counting applications use gamma sources with flux rates that range from 1 photon per second up to 105 photons per second. Some new photon counting applications are pushing the count rates over 106 counts per second. These high flux rate applications may use Bremstrahlung sources, such as conventional X-ray tubes. These sources typically supply much higher fluence than gamma sources, even at their lowest range of operation, which is in the range of 106 photons per second to up to 109 photons per second. What's more is that X-ray tubes are polychromatic sources that output a wide spectrum of energies which has a significant effect on how these photons interact with the detection material.
Direct conversion radiation detectors traditionally have been plagued by polarization effects for high count rates in photon counting mode and non-linear response in current mode. The cause of the polarization may be the result of one of the carriers, either electron or hole, having a significantly lower mobility(μ)-lifetime(τ) product (μτe—mu-tau electrons; μτh—mu-tau holes) than the other carrier. In a conventional direct conversion detector, the hole μτh can be one to two orders of magnitude less that the electron μτe.
For most conventional photon counting applications with moderate count rates, e.g., count rates of less than about 105 photons per second, polarization effects have been reduced to insignificant levels due to the extensive research that has been put into improving device fabrication techniques and carrier transport properties in the materials. Electron mobility-lifetime products of CZT, for example, have improved over two orders of magnitude over the last ten years (μτe≈7×10−3 cm2/volt). The hole mobility-lifetime product has improved over the years but still remains one to two orders of magnitude lower than μτe (μτh≈2×10−5 cm2/volt).
Hole trapping was still an issue for all photon counting applications until the recent advent of detector designs which are relatively immune to the hole motion. A detailed discussion of the “Small Pixel Effect” can be found in H. Barrett, J. D. Eskin, and H. B. Barber, Charge Transport in Arrays of Semiconductor Gamma-Ray Detectors, Physical Review Letters, vol. 75(1), p. 156, 1995.
With reference to FIG. 1, initial direct conversion radiation detectors 10 were configured as single elements having metallic “planar” contacts 12, 14 coated on both sides of the detector body 16 as shown in FIG. 1. A potential 18 is applied across these two contacts 12, 14 to establish an electric field in the bulk of the detector. This field is employed to cause the carriers to drift to their respective electrode. The carrier motion in the insulating bulk material induces a charge that is sensed in one of the electrodes connected to a charge amplifier.
Investigators noticed that for photons that deposit energy throughout the detector thickness, there was a very significant low energy tail in the measured pulse height spectra. Theory and knowledge was developed that showed that hole trapping was the cause of the tail. A Typical value for mean drift length of electrons at 1000V/cm field strength is about 2 cm, whereas, the mean drift length for holes at the same field strength is about 0.02 cm. This means that as gamma or photons interact within the detector bulk and electron-hole pairs are generated, electrons are easily swept across the full bulk of moderately thick detectors. Holes, however, have little chance of making it to their respective contacts and are usually trapped in various trap centers. With the holes trapped, the signal strength becomes dependent on the distance the electrons travel to the positive contact. This results in a depth of penetration dependent signal strength rather than just an energy dependent signal strength, which compromises the energy resolution.
As crystal growth and fabrication techniques improved, large single crystal detectors became available. For applications requiring spatial information (e.g. imaging) it became necessary to “pixelate” the detector, which meant that one large crystal could be partitioned into many sensing elements by applying an array of metallic contacts on one side of the detector. Investigator quickly noticed that pixilated devices had much better energy resolution than non-pixelated device. This phenomenon was analyzed and a theory was developed by Barrett, et al. based on the “weighting potential” concept that explained the phenomenon. The theory describes how when a detector is pixelated, and the pixel dimension is significantly smaller than the detector thickness, most of the signal induction occurs when the electrons drift in the vicinity of the pixel where the weighting field is relatively strong. The summary result is that photons or gammas can interact over a large range of depth in the detector and produce signals that have a reduced dependence on depth compared with a planar device. When the electron-hole pairs are generated, the holes are quickly trapped, but the electrons do not induce much charge at the positive contact until they get within the high weighting field region near the pixel contact where they induce most of their charge on the electrode.
With this knowledge, radiation detectors were designed with optimized ratios of pixel dimension to detector thickness to achieve improved energy resolution operating in this mode. These conventional direct conversion detectors were configured with a continuous planar cathode through which incoming photons or gammas enter the detector with the charge sensing done on the pixelated contact side. All of these architectures are irradiated from the planar side of the detector with the charge or current sensing electronics connected to the pixelated backside of the devices.
This standard small pixel mode of operation works quite well as long as the transit time is relatively short with respect to the number of events occurring per unit time. At low to medium count rates, it is typically observed in present day CZT material, that consistent pulse heights are observed over long periods of time. If trapped charge were not de-trapping at least an equilibrium rate at which new carriers were being trapped, internal fields would build up due to the accumulated charge of the trapped carriers that would polarize the bulk material. In the past, this was indeed the case for poor quality materials.
At high count rates, it is often observed in conventional detectors that the pulse height decreases with time. This is a strong indicator that traps are filling faster that they are emptying and reverse polarization is occurring. The polarization effectively reduces the field strength across the device that decreases the charge collection efficiency. This changes the signal output from the expected value and causes a non-linear response of the detector.
At very high count rates or rather high fluence, as is typical of X-ray applications with X-ray tube sources, significant decreases in signal strength occur in short time frames indicating severe polarization.